This invention relates generally to an axis position detecting system such as are employed on numerically controlled machine tools, robots or coordinate measuring machines. More specifically, this invention relates to an axis positioning system which achieves automatic and dynamic compensation of position error.
The development of high precision hydraulic and electric servo drive systems has enabled repeated positioning of the separate axis slides on present day machine tools etc. with very high accuracy. As a result, productivity has been improved because large quantities of parts can now be manufactured automatically to very precise tolerances. More recently, the servo drive technology developed previously for machine tools has been employed in the construction of robots and coordinate inspection machines. Although present day machine tools, robots and coordinate measuring systems include very sophisticated servo drive systems having very accurate feed back transducers for detecting movable member positions, some position error is inevitable.
For the most part, position error occurs in machine tools and the like because of thermal expansion. Friction between the nut and lead screw, the mechanism commonly employed to achieve precise movable member positioning, usually leads to thermal expansion of the lead screw. Thermal expansion of the lead screw results in the movable member being at a position different than that recorded by the control system controlling member movement. While temperature sensitive elements, such as a thermistor, have been employed to measure temperature variation to enable the positioning control system to effect compensation for position errors due to thermal expansion, such systems have not generally proven adequate.
Additional factors also contribute to the position error. An error in the gross fabrication of the lead screw results in a repeatable position error which is generally smaller than the thermal error but a position error nonetheless. In the past, the error attributable to deficiencies in fabrication of the lead screw has been compensated for by the use of an error table often referred to as a "laser" table.
The laser table derives its name from the device which is used to measure the actual axis slide positions, at the time the machine tool is fabricated. A laser interferometer is commonly employed to precisely calibrate each of the machine tool axes slides and for each slide, the error compensation value, if any, for each of a plurality of slide positions is stored as a separate one of the table entries together with that slide position. Thus, at any predetermined slide position, the error compensation can be obtained from the table entry of the laser table associated with that slide. While the laser table enables adequate error compensation for long distances of movable member movement, the laser table has not proven itself as an adequate mechanism for compensation of position errors which occur during short axis slide movements.
Another factor which contributes to the slide position error is the eccentricity of the pulley or gear on the lead screw and the pulley or gear on the motor which drives the lead screw. The lead screw pulley or gear eccentricity and motor pulley or gear eccentricity generally result in a sinusoidally varying position error. In addition to the error attributable to the motor pulley or gear eccentricity and lead screw pulley or gear eccentricity, some position error arises as a consequence of the eccentricity of the pulley or gear on the feedback transducer which is usually a resolver or the like. This position error also varies sinusoidally.
Most present day positioning control systems utilize a resolver as the feedback transducer. As may be appreciated, a resolver is only one type of feedback device that can be employed to provide position feedback information. The resolver, when excited with a fixed frequency signal, produces a signal whose phase shifts as the movable member moves. A zero crossing detector is usually employed to detect the phase shift to enable determination of the movable member position. The zero crossings of the resolver output signal may be erratic, especially during movable member acceleration. Also, a latency exists between the time the zero crossing occurs and the time that the zero crossing is detected by the control system. As a consequence, the actual position of the movable member will be different from that recorded in the memory of the control system.
Position errors of the type described above are very undesirable. When position errors arise in the course of machine tool operation, the machine tool accuracy is impaired. Position errors arising in the course of robot operations results in inaccurate robot arm movement which may lead to inaccurate part placement, inaccurate welding or inaccurate part measurement, depending on the robot application. Position errors are also undesirable in coordinate inspection machines as they impair the accuracy of the measurement obtained.
In contrast to the prior art, the present invention is directed to an improved positioning system which automatically and dynamically compensates for positioning errors to increase the movable member positioning accuracy.